(1) Field of the Invention
This invention lies in the field of mixed or special fuels for power plants and in particular the field of treatment by special methods, and in novel apparatus for processing solvent refined coal so that it can be used in the direct firing of power plants (with particular emphasis on gas turbine power plants) and must meet requirements for long service life and for the protection of the environment.
With regard to the generation of electric power from fossil fuel such as natural gas, petroleum fluid and coal, it is now recognized that the most efficient power plant cycle utilizes a combination of gas turbines and steam turbines in what is known as a combined cycle. The heat rates for combined cycle plants are significantly lower than those for the best steam plants in operation in the country over the entire size range for which these kinds of equipment are available. A basic characteristic of the combined cycle is that fuel is burned in a combustion chamber and the products of combustion immediately flow into the turbine where most of the energy is converted to mechanical work. Therefore, in order to utilize any possible fuel in a combined cycle plant it must be in such a form that the hot gases resulting from the combustion process are a suitable working fluid for the gas turbine portion of the set.
Although there have been many attempts to burn coal in a gas turbine, all of these have been unsuccessful because of the very high erosion resulting from the products of combustion. This erosion is the direct result of the high ash which is in the coal and is still present in the combustion products after the combustion process is completed.
(2) Description of the Prior Art
One of the solutions proposed in the prior art to solve the coal burning problem in gas turbines is to reduce the sulfur content of the coal and to also reduce the concentration of harmful elements such as vanadium and the like. The problem with corrosion is well recognized as taught in Bornstein, U.S. Pat. No. 3,581,491.
Steam treatment of coal to remove sulfur and sulfur products has been proposed in Aldridge et al., U.S. Pat. No. 3,600,013.
However, the further application of these known methods has not solved the corrosion problem and in the development of solvent refined coal the first practical fuel appears to be at hand for direct firing of power plants since it appears that sulfur and ash have been reduced to meet environmental restrictions.
In solvent refined coal, the ash content of the coal has been reduced to a sufficiently low level so that it appears the fuel has the potential for direct firing in a gas turbine with out the previous unsatisfactory erosion results, and therefore, utilization in a combined cycle power generation plant. Because of the higher efficiency of a combined cycle plant, this would clearly be a more desirable way in which to utilize this fuel for power generation purposes.
The solvent refined coal product currently being produced has one additional problem standing in the way of its being successfully utilized as a gas turbine fuel. This is that the product contains trace quantities of the elements sodium and potassium that are sufficient to promote hot sulfidation or corrosion of the high temperature gas turbine parts, particularly the first stage turbine blades. The quantities of these two elements in the solvent refined coal product as produced by current processes are in the order of 40-60 parts per million (ppm). In order for this to be a satisfactory gas turbine fuel, these trace element quantities must be reduced to a value less than 1 ppm, and this invention demonstrates that a simple non-chemical extraction process and apparatus can be made to accomplish this.
The solvent refined coal material is solid at room temperature but as delivered from the processing plant has a melting point of about 300.degree. F. It has been observed that if the material is heated to above the melting point in the presence of oxygen, or air, that an undesirable chemical reaction takes place which results in a rather rapid increase in viscosity and an increase in the melting point, however, if this heating to temperatures above the melting point takes place in the presence of an inert atmosphere, such as nitrogen or water vapor, the undesirable changes in chemical properties do not occur. The apparatus and process of the invention involves the use of water and steam and so makes it possible to further process and pump the material in a satisfactory manner.
A further advantage to the washing process of the invention is that in the solvent refined coal the total ash content is reduced as differentiated from just the sodium and postassium portion of this ash. After washing under pressure, the ash content is reduced to about 0.05% or to approximately 1/2 or less than it had in the as-received condition.
I am aware that there has been proposed in Rieves, U.S. Pat. No. 3,660,054, to heat coal with very limited quantities of water under very high pressures, e.g., about 1,545 to 3,226 psig in order to reduce the ash to about 6% and to reduce sulfur.
In contrast to this procedure in Rieves, the process of the present invention starts with solvent refined coal in which the ash content is about 0.2 to 0.1% and there are 40-60 parts per million each of sulfur and alkali metal salts such as the sodium or potassium salts.
One would not expect that ash reduction by treatment under far lower temperatures could bring about a greater than 50 fold reduction in alkali metal ash when substantial quantities remain (e.g., 6%) ash in the coal after treatment by the method of Rieves.